Controls on Dissolved Barium and Radium-226 Distributions in the Pacific Ocean Along GEOTRACES GP15

Radium-226(²²⁶Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full-depth ocean section of ²²⁶Ra activities (T_1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of ²²⁶Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher ²²⁶Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with ²²⁶Ra inputs primarily resulting from the decay of ²³⁰Th in sediments. Further, dissolved ²²⁶Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non-conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non-conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of ²²⁶Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non-conservative processes that shape their respective distributions.